Mononuclear and Binuclear Cyclopentadienyl Oxo Molybdenum and Tungsten Complexes:  Syntheses and Applications in Olefin Epoxidation Catalysis

Abstract

New half-sandwich molybdenum complexes [Mo(CpBz)Cl 2 (O)] (1), [{Mo(CpBz)(O) 2 } 2 (µ−O)] (5) and [Mo(CpBz)Cl 2 ] 2 (6) (CpBz = C 5 (CH 2 Ph) 5 ), the tungsten derivative [W(CpBz)Cl(O) 2 ] (3) and a high yield synthesis of [Mo(CpBz)Cl(O) 2 ] (2), are described. The molecular structures and cyclic voltammetry measurements of 1 and 2 and a comparative study of various molybdenum dioxo complexes as olefin epoxidation catalyst precursors are also presented. The performance of [Mo(CpPr i 4 )Cl(O) 2 ] (9) as cyclooctene epoxidation catalyst using TBHP as oxidant was determined and compared with other [MoCp'Cl(O) 2 ] complexes (Cp' = C 5 H 5 (7), C 5 Me 5 (8), CpBz (2)) to assess the activity dependence on the ring-substituents. Under the same experimental conditions, bimetallic compounds [{MoCp'(O) 2 } 2 (µ−O)] (Cp' = CpBz (5), Cp* (10), CpBu t 3 (11), CpPr i 4 (12)) have been tested and [CpMoO 2 ] 2 O was checked in aqueous solution, with H 2 O 2 and TBHP as oxidant. Finally, [Mo(CpBz)(CO) 3 CH 3 ] ( 4) was used as a catalyst precursor for cyclooctene epoxidation in the presence of TBHP. The analogy between the behaviors displayed by this complex and complex 2 is discussed.

A Computational Study of the Olefin Epoxidation Mechanism Catalyzed by Cyclopentadienyloxidomolybdenum(VI) Complexes

“…Calculations on this system may also be considered to model the action of other [Cp*MoO 2 X] catalysts (X = CH 3 or Cp*MoO 3 ). [38][39][40][41] These molecules are excellent epoxidation catalysts, provided TBHP is used as the oxidant and the solvent is apolar. [42] Chloroform is frequently used, therefore the calculations have been carried out by introducing the solvent effect by the CPCM in CHCl 3 .…”

“…[38][39][40] In a recent collaborative study, some of us showed that the dinuclear oxido-bridged [Cp # 2 Mo 2 O 5 ] systems are also catalytically active when using TBHP in an organic solvent, and are active with the same reagent under aqueous biphasic conditions. [41] However, they do not yield significant amounts of epoxidation product when TBHP is replaced by H 2 O 2 . [42] Only recently, the use of MeOOH as a model of the tBuOOH reagent has been considered in a theoretical study, based on the compound [MoO 2 Br 2 (MeN=CHCH=NMe)] as the catalyst.…”

(Pentabenzylcyclopentadienyl)molybdenum Complexes: Synthesis, Structures and Redox Properties

“…Solvent removal afforded a red microcrystalline solid in 91 % yield (1.54 g, 1.15 mmol). 1 General Procedures for X-ray Crystallography: Pertinent details for the individual compounds can be found in Tables 5 and 6. Crystallographic data for compounds 4, 6 and 7 were collected using graphite-monochromated Mo-K α radiation (λ = 0.71069 Å) with a Bruker AXS-KAPPA APEX II area detector diffractometer equipped with an Oxford Cryosystem open-flow nitrogen cryostat; data were collected at 150 K for compounds 4 and 6 and at 130 K for compound 7.…”

“…A comparison of [MoCpЈ(O) 2 Cl] (CpЈ = Cp, Cp*, CpiPr 4 , CpBz) complexes showed that the CpBz derivative leads to the most active and stable catalyst system when activated with tBuOOH. [1] Moreover, chlorination reactions of low oxidation-state (CpBz)carbonylmolybdenum and -tungsten complexes also differ from those of their Cp and Cp* analogues. In particular, the reactions of [MoCpBz(CO) 3 the application of hydrido complexes of general formula [MCp(CO) 2 L(H)] (M = Mo, W; L = CO, PR 3 ) as hydrogenation catalysts of ketones and aldehydes has recently been reported.…”

N-heterocyclic carbene complexes of Group 6 metals

“…It is a rare example of a third-row metal radical with an NHC ligand [132]. Its crystal structure shows a three legged piano stool configuration with W- -44(a,b) give similarly high activity for olefin epoxidation as CpMo(CO) 3 X (X = Cl, Br, CH 3 ) [138][139][140][141][142][143][144][145]. Such strong activity is attributed to the high stability of the Mo-C carbene bond in these ionic complexes under oxidative conditions.…”